Separating Oil and Water Streams

ABSTRACT

Embodiments described herein provide a system and methods for separating oil and water streams. The method includes separating a fluid stream into an oil continuous stream and a water continuous stream using a cyclonic separator, flowing the oil continuous stream to a first gravity separation vessel, and flowing the water continuous stream to a second gravity separation vessel. The method also includes separating the oil continuous stream in the first gravity separation vessel into an oil stream and a water stream and separating the water continuous stream in the second gravity separation vessel into an oil stream and a water stream.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the priority benefit of U.S. Provisional PatentApplication 61/537,317 filed Sep. 21, 2011 entitled SEPARATING OIL ANDWATER STREAMS the entirety of which is incorporated by reference herein.

FIELD

Exemplary embodiments of the subject innovation relate to the separationof oil and water streams in a subsea or topside environment.

BACKGROUND

Obtaining hydrocarbons from subsea environments is becoming anincreasingly important alternative to obtaining hydrocarbons fromland-based sources. As long as energy prices continue to increase, thistrend is likely to continue. Delivering hydrocarbons from a subsea wellto the surface presents technologists with a number of challenges. Waterin hydrocarbons can form hydrate clathrates in transportation lines andsubsea equipment as the fluid cools, creating flow restrictions.Further, fluid obtained from a subsea well may comprise a largeproportion of water relative to hydrocarbons, reducing the efficiency ofhydrocarbon transportation from the well. In such situations, it may bedesirable to attempt to separate hydrocarbons out of the liquid producedby a subsea well at the sea floor.

Separating hydrocarbons flowing from a subsurface well from other fluidsmay present difficulties. While subsea separation is not trivial inshallow waters, for example, fifteen hundred meters or less, it becomesmuch more challenging in deeper water. As water depth increases, theexternal pressure on a vessel created by the hydrostatic head increasesthe required wall thickness for vessels used for subsea processing. Atdepths in excess of fifteen hundred meters, this wall thickness becomesgreat enough, that typical gravity separation is not practical becausethe allowable vessel size is limited in diameter by wall thickness andweight. As a result, deepwater subsea separation of hydrocarbons isrelatively difficult because traditional large-diameter separatorscannot be used. This disadvantage is further increased if the separationis a heavy oil and water, which may emulsify.

Typically, separation of heavy oils from water necessitates the use of alarge gravity separation vessel that provides long retention times forthe oil and water to separate. However, due to size and weightconstraints, the use of large gravity separation vessels is notpractical for many applications, both on and offshore. Topsideapplications of large gravity separation vessels can be limited by thespace requirements of the vessel. In some instances, the ability to usea smaller, more efficient separation system may be desirable.

Separation of oil and water is especially difficult when the fluids arein an inversion range, for example, when the watercut of the stream inthe range between about 40% and about 60%. The watercut is the ratio ofwater produced compared to the total volume of liquid produced. In thefluid inversion range, emulsion layers may form and inhibit effectiveseparation of hydrocarbons and water. An emulsion is a physical mixtureof two liquids which are immiscible, in which one liquid exists asnearly stable droplets that are dispersed in the other liquid. Anemulsion is also known as a colloid. While overcoming this problem isdifficult topside, performance can be improved by heat, chemicals, ortime. However, in subsea applications, these techniques are often notfeasible.

The current practice to separate oil and water in the inversion range isthrough separation enhancers. An example of a separation enhancerincludes mixing chemicals, such as demulsifiers, with the fluid.However, as the fluid properties change, such as by turning on newwells, the appropriate amount of demulsifier to use becomes difficult topredict. This can lead to using excess amounts of demulsifier, which isexpensive and often causes other challenges, such as foaming.

Another example of a separation enhancer is the application of heat tolower the viscosity of the fluids and ease separation. However, applyingheat is expensive, and is very challenging in a subsea environment.

Another example of a separation enhancer is the injection of water intoa separator to raise the watercut beyond the inversion range. This isthe most common currently-used method for subsea oil and waterseparation. However, recirculation can require large amounts of water,which necessitates an increase in the size of equipment, such asvessels, pumps, and piping. Further, deepwater vessels are limited interms of available vessel volume due to the external pressures.Therefore any waste of space due to added water is a disadvantage.

Another separation enhancer involves the use of electrostaticcoalescers. Electrostatic coalescers place a charge across a fluid orfluid mixture to cause droplets of polar fluids, such as water, tocoalesce into larger droplets. Although, electrostatic coalescers arerelatively effective, and are currently used in many applications, theywill turn off automatically once the fluid mixture approaches a watercontinuous phase to avoid shorting out. Therefore, since the separationwill still be difficult past the operation point of the electrostaticcoalescers, the use of electrostatic coalescers may not be relied on asa total solution to the problem of separating oil and water in theinversion range.

U.S. Pat. No. 6,197,095 to Ditria, et al., discloses a method for subseamultiphase fluid separation. The initial step of the method is theseparation of solids using a cyclonic solids separator. In a secondstep, bulk gas is removed from the liquid using a cyclone or augerseparator. In a third step, water is separated from the oil using aliquid-liquid hydrocyclone. In a final step, a gravity separator is usedto cause further separation of the water from the oil. However, themethod is limited by the size of the gravity separator, since a fairlylarge vessel may be required to cause sufficient separation of the waterfrom the oil using one gravity separator.

International Patent Publication No. WO2004/007908 by Gulbraar, et al.,discloses an apparatus for separating water from oil. The apparatusincludes an electrostatic coalescer for the separation of the waterdroplets from the oil droplets within a stream. After the water has beenpartially separated from the oil by the electrostatic coalescer, thestream is sent to an oil/water separation arrangement for furtherseparation of the water from the oil. However, while the electrostaticcoalescer may help to avoid the formation of emulsions, the coalescermay not be sufficient to ensure the avoidance of separation in theinversion range. In addition, the use of only one oil/water separationarrangement may increase the required size of the apparatus and limitthe effectiveness of the system.

SUMMARY

An embodiment provides a method for separating oil and water streams.The method includes separating a fluid stream into an oil continuousstream and a water continuous stream using a cyclonic separator, flowingthe oil continuous stream to a first gravity separation vessel, andflowing the water continuous stream to a second gravity separationvessel. The method also includes separating the oil continuous stream inthe first gravity separation vessel into an oil stream and a waterstream and separating the water continuous stream in the second gravityseparation vessel into an oil stream and a water stream.

Another embodiment provides a system for separating oil and waterstreams. The system includes a cyclonic separator configured to separatea fluid stream into an oil continuous stream and a water continuousstream, a first gravity separation vessel configured to separate thewater continuous stream into a first oil stream and a first waterstream, and a second gravity separation vessel configured to separatethe oil continuous stream into a second oil stream and a second waterstream.

Another embodiment provides a method for separating two immisciblephases from a fluid stream. The method includes sending the fluid streaminto a cyclonic separator, generating radial acceleration within thecyclonic separator using a swirl element, and controlling the radialacceleration at a value at which the two immiscible phases separate intotwo continuous phases. The method also includes removing the twocontinuous phases from the cyclonic separator into two lines using avortex finder and sending the two continuous phases to two separatedownstream vessels for further separation of the two immiscible phases.

DESCRIPTION OF THE DRAWINGS

The advantages of the present techniques are better understood byreferring to the following detailed description and the attacheddrawings, in which:

FIG. 1 is an illustration of a subsea hydrocarbon field that uses subseaseparation techniques prior to sending materials to the surface;

FIG. 2 is a schematic of a system for separating oil and water using acyclonic separator upstream of two gravity separation vessels;

FIG. 3 is a schematic of a complete system for separating gas, oil,water, and sand;

FIG. 4 is a schematic of a complete system, including an electrostaticcoalescer, for separating gas, oil, water, and sand;

FIG. 5 is an illustrative view of a cyclonic separator that may be usedto separate oil and water streams;

FIG. 6 is an illustrative view of the swirl element that may be used inthe cyclonic separator; and

FIG. 7 is a process flow diagram showing a method for the separation ofoil and water streams.

DETAILED DESCRIPTION

In the following detailed description section, specific embodiments ofthe present techniques are described. However, to the extent that thefollowing description is specific to a particular embodiment or aparticular use of the present techniques, this is intended to be forexemplary purposes only and simply provides a description of theexemplary embodiments. Accordingly, the techniques are not limited tothe specific embodiments described below, but rather, include allalternatives, modifications, and equivalents falling within the truespirit and scope of the appended claims.

At the outset, for ease of reference, certain terms used in thisapplication and their meanings as used in this context are set forth. Tothe extent a term used herein is not defined below, it should be giventhe broadest definition persons in the pertinent art have given thatterm as reflected in at least one printed publication or issued patent.Further, the present techniques are not limited by the usage of theterms shown below, as all equivalents, synonyms, new developments, andterms or techniques that serve the same or a similar purpose areconsidered to be within the scope of the present claims.

A “facility” as used herein is a representation of a tangible piece ofphysical equipment through which hydrocarbon fluids are either producedfrom a reservoir or injected into a reservoir. In its broadest sense,the term facility is applied to any equipment that may be present alongthe flow path between a reservoir and the destination for a hydrocarbonproduct. Facilities may comprise drilling platforms, productionplatforms, production wells, injection wells, well tubulars, wellheadequipment, gathering lines, manifolds, pumps, compressors, separators,surface flow lines, and delivery outlets. In some instances, the term“surface facility” is used to distinguish those facilities other thanwells. A “facility network” is the complete collection of facilitiesthat are present in the model, which would include all wells and thesurface facilities between the wellheads and the delivery outlets.

The term “gas” is used interchangeably with “vapor,” and means asubstance or mixture of substances in the gaseous state as distinguishedfrom the liquid or solid state. Likewise, the term “liquid” means asubstance or mixture of substances in the liquid state as distinguishedfrom the gas or solid state. As used herein, “fluid” is a generic termthat may include either a gas or vapor.

A “hydrocarbon” is an organic compound that primarily includes theelements hydrogen and carbon although nitrogen, sulfur, oxygen, metals,or any number of other elements may be present in small amounts. As usedherein, hydrocarbons generally refer to organic materials that aretransported by pipeline, such as any form of natural gas or crude oil. A“hydrocarbon stream” is a stream enriched in hydrocarbons by the removalof other materials, such as water.

The terms “inversion range” or “fluid inversion range” refer to a rangebetween about 40%-60% watercut in a stream comprising water andhydrocarbons. The inversion relates to a change of phase in which thestream changes or “inverts” between a water continuous stream and an oilcontinuous stream.

“Liquefied natural gas” or “LNG” is natural gas that has been processedto remove impurities (for example, nitrogen, and water and/or heavyhydrocarbons) and then condensed into a liquid at almost atmosphericpressure by cooling and depressurization.

The term “natural gas” refers to a multi-component gas obtained from acrude oil well (termed associated gas) or from a subterraneangas-bearing formation (termed non-associated gas). The composition andpressure of natural gas can vary significantly. A typical natural gasstream contains methane (CH₄) as a significant component. Raw naturalgas will also typically contain ethylene (C₂H₄), ethane (C₂H₆), otherhydrocarbons, one or more acid gases (such as carbon dioxide, hydrogensulfide, carbonyl sulfide, carbon disulfide, and mercaptans), and minoramounts of contaminants such as water, nitrogen, iron sulfide, wax, andcrude oil.

“Pressure” is the force exerted per unit area by the fluid on the wallsof the volume. Pressure can be shown as pounds per square inch (psi).“Atmospheric pressure” refers to the local pressure of the air.“Absolute pressure” (psia) refers to the sum of the atmospheric pressure(14.7 psia at standard conditions) plus the gage pressure (psig). “Gaugepressure” (psig) refers to the pressure measured by a gauge, whichindicates only the pressure exceeding the local atmospheric pressure(i.e., a gauge pressure of 0 psig corresponds to an absolute pressure of14.7 psia).

“Production fluid” refers to a liquid and/or gaseous stream removed froma subsurface formation, such as an organic-rich rock formation. Producedfluids may include both hydrocarbon fluids and non-hydrocarbon fluids.For example, production fluids may include, but are not limited to, oil,natural gas, and water.

“Substantial” when used in reference to a quantity or amount of amaterial, or a specific characteristic thereof, refers to an amount thatis sufficient to provide an effect that the material or characteristicwas intended to provide. The exact degree of deviation allowable may insome cases depend on the specific context.

The term “watercut” refers to the proportion of water present in astream that comprises both water and other components such ashydrocarbons. For example, a stream having a 20% watercut comprisesabout 20% water and 80% other components.

“Well” or “wellbore” refers to a hole in the subsurface made by drillingor insertion of a conduit into the subsurface. The terms areinterchangeable when referring to an opening in the formation. A wellmay have a substantially circular cross section, or othercross-sectional shapes (for example, circles, ovals, squares,rectangles, triangles, slits, or other regular or irregular shapes).Wells may be cased, cased and cemented, or open-hole well, and may beany type, including, but not limited to a producing well, anexperimental well, and an exploratory well, or the like. A well may bevertical, horizontal, or any angle between vertical and horizontal (adeviated well), for example a vertical well may comprise a non-verticalcomponent.

“Clathrate hydrates” (hereinafter clathrate or hydrate) are compositesformed from a water matrix and a guest molecule, such as methane orcarbon dioxide, among others. Clathrates may form, for example, at thehigh pressures and low temperatures that may be found in pipelines andother hydrocarbon equipment. For any particular clathrate compositioninvolving water and guest molecules, such as methane, ethane, propane,carbon dioxide, and hydrogen sulfide, at a particular pressure there isa specific clathrate equilibrium temperature, above which clathrates arenot stable and below which they are stable. After forming, theclathrates can agglomerate, leading to plugging or fouling of theequipment. Further, many hydrocarbons, such as crude oil, may containsignificant amounts of wax, e.g., in the form of paraffinic compoundsthat may precipitate as temperatures are lowered. These paraffiniccompounds can form layers along cold surfaces, such as the inner wall ofa subsea pipeline, and can cause fouling or plugging of equipment.

A “piston motor valve” (PMV) is a type of valve that uses the linearmotion of a piston to open or close the valve. A PMV is used when afully open or fully closed valve is desirable for flow control.

A “diaphragm motor valve” (DMV) is a type of device or component thatmay be used to control the flow of a fluid through a pipe or tube bymoving a valve though a range of positions from fully closed to fullyopen. A DMV is generally used to throttle fluid flow in a line.

Overview

Current separation systems, including both subsea and topside separationsystems, encounter difficulty in the separation of oil and water oncethe fluid mixture approaches the inversion range. The inversion range ofan oil and water fluid mixture is typically around 40% to 60% watercut.Below the lower watercut limit, the mixture is usually oil continuous;and the water droplets are dispersed into the oil. Above the upperwatercut limit, the mixture is usually water continuous and the oil isthe dispersed phase. However, emulsions can be formed in theintermediate range of watercuts where the two phases are switching fromcontinuous to dispersed phase. The formation of stable emulsions makesseparation of oil and water in the inversion range very challenging.

Embodiments disclosed herein provide methods and systems that allow forthe separation of gas, oil, water, and sand throughout all watercuts,including those in the inversion range. The method lowers the likelihoodof separating mixtures in the inversion range through the utilization ofa cyclonic separator upstream of two gravity separation vessels.Accordingly, the system may function effectively without the use ofseparation enhancers for gravity separation in the fluid inversionrange, since gravity separation techniques are not applied while thefluid is in the inversion range.

The cyclonic separator is placed upstream of the two gravity separationvessels to provide an initial separation of the fluid into a watercontinuous stream and an oil continuous stream. A fixed swirl elementinside the cyclonic separator creates a radial acceleration in thefluid, e.g., by generating a cyclone action. The fixed swirl element isdesigned to have a low pressure drop to lower turbulence and fluidmixing. The cyclonic action generates a centripetal force that causesthe heavier fluid phase, water, to move towards the outside wall of thepipe and the lighter fluid phase, oil, to move towards the center. Inthis manner, the swirl element performs a bulk phase separation of thefluids.

The streams may be split by means of a vortex finder in the center ofthe cyclonic separator or by a horizontal flow split with a bafflerunning parallel to the cyclonic separator. Some amount of the othercomponent will remain in each stream. The two different streams may besent to separate gravity separation vessels, e.g., pipe separators, forfurther separation of oil and water phases. Thus, separation of fluidsin a gravity separation vessel is not attempted while the mixture is inthe inversion range.

After the two different streams enter into the separate gravityseparation vessels, further separation is performed, and each vessel hasan oil outlet and a water outlet. The oil streams from the gravityseparation vessels may remain separate or may be blended together. Thesame is true for the water streams from the gravity separation vessels.The particular application or system configuration may be used todetermine whether the fluid streams from multiple gravity separationvessels should be combined or remain separated. Further, a number of theseparation systems may be utilized in one area, with the like streamscombined into single streams.

In an embodiment, the present techniques may be used in anytransportation or production environment that is susceptible toclathrate, including subsea to shore pipelines, on-shore pipelines,wells, oil from oil sands, natural gas, or any number of liquid orgaseous hydrocarbons from any number of sources. For example, a specificapplication of the present techniques may include the protection ofsubsea lines from a production field.

The system described herein may be used on the seafloor to separate oiland water mixtures that are in or near the inversion range by avoidinggravity separation while fluids are in the inversion range. This systemdoes not depend on the use of separation enhancers, which can be costlyand may limit the capacity of the system.

FIG. 1 is an illustration of a subsea hydrocarbon field 100 that uses acyclonic separation technique. The field 100 can have a number ofwellheads 102 coupled to wells 104 that harvest hydrocarbons from aformation (not shown). As shown in this example, the wellheads 102 maybe located on the ocean floor 106. Each of the wells 104 may includesingle wellbores or multiple, branched wellbores. Each of the wellheads102 can be coupled to a central pipeline 108 by gathering lines 110. Thecentral pipeline 108 may continue through the field 100, coupling tofurther wellheads 102, as indicated by reference number 112.

In an embodiment, a cyclonic separation system 114 is used for theseparation of gas, oil, water, and sand from the central pipeline 108.Three lines 116, 118, and 120 may couple the cyclonic separation system114 to a platform 122 at the ocean surface 124. The three lines 116,118, and 120 may be flexible to allow movement of the platform 122. Theflexible lines 116, 118, and 120 may carry gas, oil, and water,respectively, to the platform 122. The platform 122 may be, for example,a floating processing station, such as a floating storage and offloadingunit (or FSO), that is anchored to the sea floor 106 by a number oftethers 126.

Any number of other types of platforms or rigs may be used. For example,the platform 122 may be a production platform with equipment fordehydration, purification, oil and water separation, oil and gasseparation, and the like, such as a storage vessel or separation vessel128. The platform 122 may be a drilling platform that includes drillingequipment, such as a tower or derrick 130. The platform 122 maytransport the processed hydrocarbons to shore facilities by pipeline(not shown). The separation of the hydrocarbons in a cyclonic separationsystem 114 may prevent the formation of hydrate plugs in transportationlines to the surface, as the oil lines 118, and gas lines 118, areseparate from the water lines 120. Further, the separation at the seafloor, or at a well site in a surface field, may provide water forreinjection into the formation to enhance production.

Cyclonic Separation Apparatus

FIG. 2 is a schematic of a system 200 for separating oil and water usinga cyclonic separator 202 upstream of two gravity separation vessels 204and 206. The cyclonic separator 202 is discussed further with respect toFIG. 5. The cyclonic separator 202 produces an oil continuous stream 208and a water continuous stream 210. The concentration of oil and water,respectively, in each of the two streams may remain above 60%.Therefore, the two streams may remain outside of the inversion range andbe easier to separate.

In an embodiment, the flow of liquid out of the cyclonic separator 202may be controlled by a level control or a back pressure control, or anycombination thereof, located on the cyclonic separator 202. The levelcontrol or back pressure control may allow for the extraction of theappropriate amount of liquid from the cyclonic separator 202 accordingto the application or the operation of the gravity separation vessels204 and 206 downstream of the cyclonic separator 202. In anotherembodiment, any other type of controls may be used in conjunction withthe system 200 to regulate the flow of liquid from one component of thesystem 200 to another.

The oil continuous stream 208 flows into a first gravity separationvessel 204, and the water continuous stream 210 flows into a secondgravity separation vessel 206. The cyclonic separator 202 separatesmixtures of oil and water that may be in the inversion range prior tothe gravity separation by the gravity separation vessels 204 and 206.

Once the oil continuous stream 208 has entered the gravity separationvessel 204, the oil and the water within the fluid may be separatedusing conventional gravity separation techniques. The less dense oil mayfloat to the top and exit as oil stream 212, while the denser water maysink to the bottom and exit as water stream 214. The same process mayoccur once the water continuous stream 210 has entered the gravityseparation vessel 206. The oil may exit as oil stream 216 at the top ofthe gravity separation vessel 206, and the water may exit as waterstream 218 at the bottom of the vessel. The pressure, fluid level, andtemperature within the gravity separation vessels 204 and 206 may bemonitored using sensors 220, 222, and 224, respectively.

The oil streams 212 and 216 containing the oil continuous phase may bejoined into one oil stream 226, while the water streams 214 and 218containing the water continuous phase may be joined into one waterstream 228. Diaphragm motor valves (DMV) 230 may be used as controlvalves to adjust the amount of flow of streams 226 and 228. For example,the DMVs 230 may be partially opened or partially closed to adjust thevelocity and pressure of the streams 226 and 228. In addition, theoil-in-water concentration of the stream 228 may be monitored by an OIWsensor 232. The amount of oil in the water continuous phase at any pointin time may be used to determine the appropriate action to take withrespect to the fluid. For example, if a large amount of oil is left inthe water continuous phase, the fluid may be passed through anothercyclonic separator, or de-oiler, to salvage as much oil as possible.

The system 200 may also include additional control features. Forexample, a fluid level sensor 222 may determine the fluid level in thepipes and send the information regarding the fluid level to a computingdevice. The computing device may include a programmable logic controller(PLC), distributed control system (DCS), or a direct digital controller(DDC), among others. The computing device may use the fluid levelinformation to control the DMV 226, as indicated by the dotted line 234.The DMV 226 may act as an effector by adjusting the position of thevalve to allow for increased or decreased fluid flow. In anotherembodiment, the DMV 226 may be a smart valve, which may act as both thecomputing device and the effector. In addition, it should be noted thatmultiple DMVs or other effectors, for example, pumps or PMVs, may beconnected to one computing device and controlled based on changes tomultiple different sensors.

In an embodiment, any number of additional gravity separation vesselsmay be used in conjunction with the system 200. For example, the gravityseparation vessels 204 and 206 may be configured to operate in parallelwith two additional gravity separation vessels to allow for a higherdegree of separation of the oil from the water. As another example, twoadditional gravity separation vessels may be configured to operate inseries with and downstream of the gravity separation vessels 204 and206.

In another embodiment, additional cyclonic separators may be useddownstream of the first cyclonic separator 202. The additional cyclonicseparators may be placed upstream or downstream of the gravityseparation vessels 204 and 206, or may replace the gravity separationvessels 204 and 206. In yet another embodiment, the cyclonic separator202 may be replaced with a bundle of multiple cyclonic separatorsarranged in series or parallel. The use of multiple cyclonic separatorsmay allow for a more efficient separation process.

FIG. 3 is a schematic of a complete system 300 for separating gas, oil,water, and sand. In FIG. 3, like numbered items are as discussed withrespect to FIGS. 1 and 2. Liquid may flow into the system through aninitial control valve 302 from the central pipeline 108. A series ofsensors 304, 306, 308, and 310 may be used to measure the fluidpressure, temperature, multiphase flow rate, and sand content,respectively, as the fluid flows through the initial control valve 302and into the gas liquid separator 312. The gas liquid separator 312 maybe used for bulk separation of the gas phase from the liquid phase. Thepressure, temperature, and fluid levels within the gas liquid separatormay be monitored using sensors 314, 316, and 318, respectively. Once thegas phase and the liquid phase have been separated by the gas liquidseparator 312, the gas may flow out as gas stream 320, and the liquidmay flow out as liquid stream 322.

A DMV 323 may be controlled by the feedback from the sensors 314, 316,and 318. The feedback may be used to determine whether the DMV 323should be opened, closed, or partially opened or closed, as indicated bythe dotted line 324. The flow of the gas stream 320 may also bemonitored using a sensor 326. When the DMV 323 is open, the gas stream320 may flow into the gas polisher 328. The gas polisher 328 may be usedto purify the natural gas. The differential pressure within the gaspolisher 328 may be monitored using a differential pressure sensor 330.The value measured by the differential pressure sensor 330 may be usedto control the position of the DMV 332, which controls the flow of anoutlet gas stream 334, as indicated by the dotted line 336. When the DMV332 is open, the gas stream 334 from the gas polisher 328 may be sent tothe collection platform 122 (not shown), as discussed with respect toFIG. 1. In addition, the flow of the gas stream 334 may be controlled byan orifice plate 335. The orifice plate 335 may be used to control thepressure of gas stream 334 in order to reduce the possibility ofbackflow of the gas stream 334 into any downstream lines. It should benoted that any number of additional orifice plates 335 may be locatedwithin the system 300 and may be used to control the pressure of variousstreams within the system 300.

A level sensor 338 may also be used to measure the fluid level withinthe gas polisher 328 and may be used to control a DMV 340, as indicatedby the dotted line 342. When the DMV 340 is open, the liquid that hasbeen separated from the natural gas by the gas polisher 328 may flowinto the system 200 as liquid stream 344. The liquid stream 344 may becoupled to liquid stream 346 as it enters system 200.

The flow rate of the liquid stream 322 from the gas liquid separator 312may be monitored by a sensor 348. The liquid stream 322 may flow into adesander 350, for example, to cyclonically separate the sand from theliquid stream 322. A DMV 352 may be used to control the outflow of sandfrom the desander 350 as stream 354. The DMV 352 may be controlled byfeedback from a differential pressure sensor 356, which measures thedifferential pressure between streams 322, 346, and 354. When the DMV352 is open, stream 354 may flow into a sand accumulator 358.

From the sand accumulator 358, the sand may take several routes. Thesand may be released as stream 360 through a PMV 362. The sand may beejected as stream 360 into the outflowing water stream. The PMV 362 maybe either entirely open or entirely closed, depending on thespecification of the operator or specific parameters of system 300. Thesand content and pressure within the sand accumulator 358 may bemonitored using sensors 364 and 366, respectively. The values measuredby the sensors 364 and 366 may be used to control the position of PMV362.

In addition, the DMV 367 may be used to control the flow of sand out ofthe sand accumulator 358. In an embodiment, the DMV 367 may remainclosed as the sand accumulator 358 becomes pressurized as it fills withsand. Once the sand accumulator 358 has reached a certain pressurelevel, the DMV 367 may open to allow the sand accumulator 358 to beemptied.

For a safety measure, a stream 368 may be allowed to flow out of the topof the sand accumulator 358 if the sand accumulation level becomes toohigh. The primary purpose of stream 368 is to prevent the failure ofsystem 300 through the clogging of the sand accumulator 358 in the caseof the failure of PMV 362. In addition, any remaining liquid in the sandaccumulator 358 may be released as stream 370 through a PMV 372. In anembodiment, stream 370 may include the liquid (mostly water) from whichthe sand has settled and may be released from the top of the sandaccumulator 358. Stream 370 may allow for the maintenance of massbalance within the sand accumulator 358 as stream 354 flows into theaccumulator 358.

The liquid stream 346 from desander 350 may flow out as to be input intothe separation cyclone 202. A DMV 374 may regulate the flow of liquidstream 346 into the system 200. The DMV 374 may be controlled usingfeedback from the level sensor 318, which determines the fluid levelwithin the gas liquid separator 312, as indicated by the dotted line376.

The cyclone 202 may receive incoming liquid streams 344 and 346. Thesystem 200 may be used to separate the oil from the water, as discussedwith respect to FIG. 2. The cyclonic separator 202 may be used to createtwo fluid streams. The cyclonic separator 202 may send an oil continuousstream 208 to the gravity separation vessel 204 and a water continuousstream 210 to the gravity separation vessel 206. The oil from thegravity separation vessels 204 and 206 may be collected into oil streams212 and 216, while the water may be collected into water streams 214 and218, respectively.

As discussed with respect to FIG. 2, the pressure, fluid level, andtemperature within the gravity separation vessels 204 and 206 may bemonitored using sensors 220, 222, and 224, respectively. The fluid levelinformation may be used to control the position of the DMV 230, asindicated by the dotted line 234. In addition, the information from thesensors 220, 222, and 224 may be used to control the positions ofvarious other control valves, including DMV 376 and DMV 378, asindicated by dotted lines 380 and 382, respectively. The DMV 376 maycontrol the flow of stream 384 into the gravity separation vessel 204.The DMV 378 may control the flow of a gas stream 386 from the gravityseparation vessel 204 to the gas outlet stream 334. An additional stream388 may also be directed to the gravity separation vessel 204 fromstream 368. The flow of stream 388 may be controlled by the DMV 390. Thestream 388 may include any fluid that was remaining the in the stream368.

Once the oil streams 212 and 216 are combined into one oil stream 226,the DMV 230 may control the flow of the oil stream 226. When the DMV 230is open, the oil stream 226 may flow to an oil pump 392. After the oilhas passed through the oil pump 392, it may be flow as stream 394 to theplatform 122 (not shown), as discussed with respect to FIG. 1.

Once the water streams 214 and 218 are combined into one water stream228, the DMV 230 may control the flow of the water stream 228. When theDMV 230 is open, the water stream 228 may flow into a bulk de-oiler 396.The bulk-de-oiler 396 may be a type of cyclonic separator that is usedto separate oil droplets from water droplets. Any remaining oil in thewater stream 228 may be separated from the water by the bulk de-oiler396 and sent as oil stream 398 to be combined with the main oil stream226 upstream of the oil pump 392. The flow of oil stream 398 may becontrolled by the DMV 400. Once the water has been separated from theoil in the bulk de-oiler 396, the water stream 402 may be sent to asecond de-oiler 404. In addition, the differential pressure between oilstream 398 and water stream 402 may be measured by the differentialpressure sensor 406. The differential pressure value measured by thesensor 406 may be used as feedback to control the DMV 400, as indicatedby the dotted line 408. The DMV 400 may be used to control the flow ofoil stream 398.

The second de-oiler 404 may be used to ensure an even higher degree ofoil and water separation by repeating the separation process one moretime. The oil which is separated from the water within the secondde-oiler 404 may be sent as oil stream 410 to be combined with oilstream 398. A differential pressure sensor 412 may measure thedifferential pressure between oil streams 402 and 410 and value of themeasurement may be used to control the position DMV 414, as indicated bydotted line 416. The DMV 414 may control the flow of oil stream 410.Once the remaining oil has been separated from the water by the secondde-oiler 404, the water stream 418 may be sent to a water injection pump420. The sensor 422 may be used to measure the final oil-in-waterconcentration of water stream 418. In addition, water stream 370 fromthe sand accumulator 358 may flow into water stream 418. In anembodiment, if the oil-in-water concentration is considered to be lowenough, the water injection pump 420 may be omitted, and the waterstream 418 may be released into the ocean. In some circumstances,additional purification of the water may also take place beforereleasing the water stream 418 into the ocean. In another embodiment,the water stream 424 exiting the injection pump 420 may be sent to theplatform 122, as discussed with respect to FIG. 1. The sand streams 360and 368 may also flow into stream 424 to be injected, released into theocean, or sent to the platform 122 (not shown).

It should be noted that the system 300 is not limited to theconfiguration shown but, instead, may be arranged in any number ofdifferent ways using any number of different components. For example,additional DMVs, PMVs, and sensors may be added to the system 300 toimprove the functioning of the system 300. As another example, morede-oilers may be added to the system in addition to the de-oilers 396and 404.

FIG. 3 illustrates a four-phase separation system that may achieve theseparation of gas, oil, water, and sand on the sea floor. The majorityof the components of system 300 may be designed based on pipe code suchthat the required wall thicknesses are greatly reduced while still beinguseful in deepwater operations. In addition, the use of system 300 as asubsea separation system may allow for the separation of oil and waterin the inversion range without the use of separation enhancers, whichare costly and may limit capacity.

FIG. 4 is a schematic of a complete system 426, including anelectrostatic coalescer 428, for separating gas, oil, water, and sand.The system 426 is the same as system 300, except for the addition of theelectrostatic coalescer 428 upstream of the cyclonic separator 202.Thus, in FIG. 4, like numbers are as discussed with respect to theprevious figures. An “electrostatic coalescer” is a device that may beused to separate an emulsion into its components, e.g., water and oil,in this case, by subjecting the emulsion to a high-voltage electricalfield. The electrical field causes the water droplets in the emulsion,which are conductive, to separate from the oil droplets, which arenon-conductive, by combining. The separation of the water from the oilin the fluid may help to avoid the inversion range.

As shown in FIG. 4, the electrostatic coalescer 428 may be positionedimmediately upstream of the cyclonic separator 202. The location of theelectrostatic coalescer upstream of the cyclonic separator 202 mayenhance the coalescence and separation of the dispersed phase, e.g.,water, in the fluid. While the radial acceleration of the swirl elementwithin the cyclonic separator 202 may be sufficient for separating thewater and oil phases, the use of the electrostatic coalescer 428 may bebeneficial, particularly in the case of emulsion formation or thepresence of high-viscosity oil within the fluid.

In another embodiment, the electrostatic coalescer 428 may be positioneddownstream of the cyclonic separator 202 and upstream of the gravityseparation vessel 204. In this case, the electrostatic coalescer 428 maybe used for the same purpose as in the previous embodiment. However, itshould be noted that an electrostatic coalescer 428 will turn offautomatically if the fluid mixture approaches water continuity in orderto avoid a short out and to conform to safety standards. Therefore, anelectrostatic coalescer may not be positioned upstream of gravityseparation vessel 206, since the water continuous stream 210 flows intogravity separation vessel 206.

Cyclonic Separator

FIG. 5 is an illustrative view of a cyclonic separator 500 that may beused to separate oil and water streams. In FIG. 5, like numbers are asdiscussed with respect to the previous figures. The stream may enter thevessel 502 of the cyclonic separator 500 as stream 346, as discussedwith respect to FIGS. 2 and 3. As the fluid enters the vessel 502, aswirl element 504 within the vessel 502 may impart a radial accelerationand a tangential velocity component to the fluid through the rotation oftwisted swirl vanes. The swirl vanes may be arranged parallel or inseries on the swirl element 504. The swirling of the fluid using theswirl element may be controlled to maintain the radial acceleration at avalue at which the two phases separate into two continuous phases, whileminimizing the turbulence to avoid shearing of the fluid. If shearingoccurs within the fluid, an emulsion of oil and water may form. Once anemulsion has formed, it becomes even more challenging and costly toseparate the oil and water. Therefore, in order for the cyclonicseparator 202 to be effective, the centrifugal force that is generatedshould be enough to effect bulk separation of the oil and water, but notenough to cause significant shearing effects within the fluid. Tominimize the shearing, the radial acceleration of the fluid may bemaintained at a value which does not cause a total pressure dropexceeding 1 bar in the fluid.

The radial acceleration imparted to the fluid may cause the fluid tobegin swirling through the vessel 502 due to the generated centrifugalforce. The heavier and denser water droplets may migrate to the outerrim of the vessel 502 and begin traveling in a wide circular path, whilethe lighter and less dense oil droplets may migrate towards the centerof the vessel 502 and begin traveling in a narrow circular path. As thefluid continues to move through the vessel 502, the fluid may beseparated into two phases, an oil continuous phase, and a watercontinuous phase. As the fluid nears the end of the vessel 502, a vortexfinder 506 may be used to capture the oil continuous phase and send itout as oil stream 208, while an outlet 508 may be used to capture thewater continuous phase and send it out as water stream 210.

An antiswirl device (not shown) may be positioned downstream of thecyclonic separator 500. The antiswirl device may be used to reduce thetangential velocity component of the oil stream 208 or the water stream210 perpendicular to the flow path. The antiswirl device may help toalign the flow path of a stream before the fluid enters a gravityseparation vessel, lessening the likelihood the tangential velocity maycause mixing in the gravity separation vessel.

FIG. 6 is an illustrative view 600 of a swirl element 504 that may beused in the cyclonic separator 500. In FIG. 6, like numbers are asdiscussed with respect to the previous figures. The swirl element 504may be affixed inside the cyclonic separator pipe 502, near the inlet ofstream 346. The swirl element may include several twisted swirl vanes602 that are used to swirl the stream 346 within the cyclonic separatorpipe 502. The swirl vanes 602 may be arranged parallel or in series onthe swirl element 504 and may be positioned at a particular angularorientation in order to effectively control the swirling of the fluid.The radial acceleration may be maintained at a value which causes theseparation of the two phases while preventing the generation of shearingforces within the fluid. If shearing of the fluid occurs, an emulsionmay form. Emulsion generation may make it more difficult to separate twophases, due to the strong interaction forces between the individualparticles of the two phases.

As the fluid flow rotates downstream of the swirl element 602, the oilcontinuous phase, indicated by the dark area in FIG. 6, moves to thecore of the cyclonic separator pipe 502, while the water continuousphase, indicated by the light area in FIG. 6, moves toward the outerwall of the pipe 502. The swirl element 504 creates a gentle rotationwithin the fluid, thereby utilizing the centrifugal force of therotation to move the heavier, denser water droplets within the fluidtoward the outer wall. The ultimate effect is to increase the number ofwater droplet interactions and oil droplet interactions and, thus,coalescing the droplets in the stream and removing the water phase fromthe oil phase.

Method for Separation

FIG. 7 is a process flow diagram showing a method 700 for the separationof oil and water streams. The method 700 may be useful for theharvesting of hydrocarbons from an oil well in both subsea and topsideenvironments. In addition, method 700 may separate oil and water streamsefficiently by avoiding the gravity separation of the two phases in theinversion range.

At block 702, the stream may be separated into an oil continuous streamand a water continuous stream using a cyclonic separator. The cyclonicseparator may use a number of swirl vanes arranged parallel or in seriesto generate radial acceleration within the stream, as discussed withrespect to FIGS. 5 and 6. The radial acceleration within the cyclonicdevice may also be controlled to ensure effective separation of the oilcontinuous phase and the water continuous phase.

At block 704, the oil continuous stream may be allowed to flow into afirst gravity separation vessel. The oil continuous stream may bedirected from the cyclonic separator to the first gravity separationvessel using a vortex finder extended through the center of the cyclonicseparator pipe. An antiswirl device may be used to straighten the flowof the oil continuous stream upstream of the first gravity separationvessel.

At block 706, the water continuous stream may be allowed to flow into asecond gravity separation vessel. The water continuous stream may bedirected from the cyclonic separator to the second gravity separationvessel through an outlet on the bottom of the cyclonic separator pipe.The outlet may capture the water continuous stream as it flows in a widecircular path around the rim of the cyclonic separator pipe. Anantiswirl device may be used to straighten the flow of the watercontinuous stream upstream of the second gravity separation vessel.

At block 708, the oil may be separated from the water in the firstgravity separation vessel using gravity separation techniques. Becausewater is heavy and denser than oil, the water will settle at the bottomof the vessel, while the oil will float to the top. At block 710, theoil may be separated from the water in the second gravity separationvessel using the same gravity separation techniques as those discussedwith respect to block 708.

It should be noted that the process flow diagram is not intended toindicate that the steps of method 700 must be executed in any particularorder or that every step must be included for every case. Further,additional steps may be included which are not shown in FIG. 7. Forexample, the two oil streams may be combined into a single oil stream,and the two water streams may be combined into a single water streamdownstream of the first and second gravity separation vessels.

While the present techniques may be susceptible to various modificationsand alternative forms, the exemplary embodiments discussed above havebeen shown only by way of example. However, it should again beunderstood that the technique is not intended to be limited to theparticular embodiments disclosed herein. Indeed, the present techniquesinclude all alternatives, modifications, and equivalents falling withinthe true spirit and scope of the appended claims.

Embodiments

Embodiments of the invention may include any of the following methodsand systems, among others, as discussed herein. This is not to beconsidered a complete listing of all possible embodiments, as any numberof variations can be envisioned from the description above.

An exemplary embodiment provides a method for separating oil and waterstreams. The method includes separating a fluid stream into an oilcontinuous stream and a water continuous stream using a cyclonicseparator, flowing the oil continuous stream to a first gravityseparation vessel, and flowing the water continuous stream to a secondgravity separation vessel. The method also includes separating the oilcontinuous stream in the first gravity separation vessel into an oilstream and a water stream and separating the water continuous stream inthe second gravity separation vessel into an oil stream and a waterstream.

In some embodiments, the method may include combining the oil streamsinto a single oil stream and combining the water streams into a singlewater stream.

In some embodiments, the method may include using a swirl element withinthe cyclonic separator to impart radial acceleration to the fluidstream.

In some embodiments, the method may include controlling a radialacceleration to avoid forming an emulsion.

In some embodiments, the method may include controlling the radialacceleration using a plurality of swirl vanes arranged in parallel or inseries on the swirl element.

In some embodiments, the method may include generating the radialacceleration within the fluid stream with a total pressure drop of lessthan about 1 bar.

In some embodiments, the method may include using a vortex finder withinthe cyclonic separator to remove the oil continuous stream.

In some embodiments, the method may include using an electrostaticcoalescer upstream of the cyclonic separator to create larger waterdroplets.

In some embodiments, the method may include using an electrostaticcoalescer downstream of the cyclonic separator and upstream of the firstgravity separation vessel.

In some embodiments, the method may include automatically shutting offthe electrostatic coalescer if the fluid stream approaches a watercontinuous phase.

In some embodiments, the method may include using an additional cyclonicseparator downstream of the first gravity separation vessel or thesecond gravity separation vessel, or both, for further separation of oilfrom water.

Another exemplary embodiment provides a system for separating oil andwater streams. The system includes a cyclonic separator configured toseparate a fluid stream into an oil continuous stream and a watercontinuous stream, a first gravity separation vessel configured toseparate the water continuous stream into a first oil stream and a firstwater stream, and a second gravity separation vessel configured toseparate the oil continuous stream into a second oil stream and a secondwater stream.

In some embodiments, the system includes an electrostatic coalescerupstream of the cyclonic separator.

In some embodiments, the system includes an electrostatic coalescer onthe oil continuous stream.

In some embodiments, the system includes a swirl element within thecyclonic separator comprises a plurality of swirl vanes arrangedparallel or in series.

In some embodiments, the system includes an antiswirl device forstraightening a flow path of the water continuous stream or the oilcontinuous stream, or both, downstream of the cyclonic separator.

Another exemplary embodiment provides a method for separating twoimmiscible phases from a fluid stream. The method includes sending thefluid stream into a cyclonic separator, generating radial accelerationwithin the cyclonic separator using a swirl element, and controlling theradial acceleration at a value at which the two immiscible phasesseparate into two continuous phases. The method also includes removingthe two continuous phases from the cyclonic separator into two linesusing a vortex finder and sending the two continuous phases to twoseparate downstream vessels for further separation of the two immisciblephases.

In some embodiments, the method includes controlling the radialacceleration of the fluid stream by selecting an angular orientation ofat least one swirl vane on the swirl element.

In some embodiments, the method includes decreasing the tangentialvelocity component of the fluid stream perpendicular to a flow pathusing an antiswirl device downstream of a point at which the radialacceleration was generated.

In some embodiments, the method includes controlling the swirling of thefluid stream to maintain the radial acceleration at a value at whichshearing of the two immiscible phases does not cause an emulsion toform.

1. A method for separating oil and water streams, comprising: separatinga fluid stream into an oil continuous stream and a water continuousstream using a cyclonic separator; flowing the oil continuous stream toa first gravity separation vessel; flowing the water continuous streamto a second gravity separation vessel; separating the oil continuousstream in the first gravity separation vessel into a first oil streamand a first water stream; and separating the water continuous stream inthe second gravity separation vessel into a second oil stream and asecond water stream.
 2. The method of claim 1, further comprising:combining the first oil stream and the second oil stream into a singleoil stream; and combining the first water stream and the second waterstream into a single water stream.
 3. The method of claim 1, comprisingusing a swirl element within the cyclonic separator to impart radialacceleration to the fluid stream.
 4. The method of claim 2, comprisingcontrolling a radial acceleration to avoid forming an emulsion.
 5. Themethod of claim 4, comprising controlling the radial acceleration usinga plurality of swirl vanes arranged in parallel or in series on theswirl element.
 6. The method of claim 3, comprising generating theradial acceleration within the fluid stream with a total pressure dropof less than about 1 bar.
 7. The method of claim 1, comprising using avortex finder within the cyclonic separator to remove the oil continuousstream.
 8. The method of claim 1, comprising using an electrostaticcoalescer upstream of the cyclonic separator to create larger waterdroplets.
 9. The method of claim 1, comprising using an electrostaticcoalescer downstream of the cyclonic separator and upstream of the firstgravity separation vessel.
 10. The method of claim 8, comprisingautomatically shutting off the electrostatic coalescer if the fluidstream approaches a water continuous phase.
 11. The method of claim 1,comprising using an additional cyclonic separator downstream of thefirst gravity separation vessel or the second gravity separation vessel,or both, for further separation of oil from water.
 12. A system forseparating oil and water streams, comprising: a cyclonic separatorconfigured to separate a fluid stream into an oil continuous stream anda water continuous stream; a first gravity separation vessel configuredto separate the water continuous stream into a first oil stream and afirst water stream; and a second gravity separation vessel configured toseparate the oil continuous stream into a second oil stream and a secondwater stream.
 13. The system of claim 12, comprising an electrostaticcoalescer upstream of the cyclonic separator.
 14. The system of claim12, comprising an electrostatic coalescer on the oil continuous stream.15. The system of claim 12, wherein a swirl element within the cyclonicseparator comprises a plurality of swirl vanes arranged parallel or inseries.
 16. The system of claim 12, comprising an antiswirl device forstraightening a flow path of the water continuous stream or the oilcontinuous stream, or both, downstream of the cyclonic separator.
 17. Amethod for separating two immiscible phases from a fluid stream,comprising: sending the fluid stream into a cyclonic separator;generating radial acceleration within the cyclonic separator using aswirl element; controlling the radial acceleration at a value at whichthe two immiscible phases separate into two continuous phases; removingthe two continuous phases from the cyclonic separator into two linesusing a vortex finder; and sending the two continuous phases to twoseparate downstream vessels for further separation of the two immisciblephases.
 18. The method of claim 17, comprising controlling the radialacceleration of the fluid stream by selecting an angular orientation ofat least one swirl vane on the swirl element.
 19. The method of claim17, comprising decreasing the tangential velocity component of the fluidstream perpendicular to a flow path using an antiswirl device downstreamof a point at which the radial acceleration was generated.
 20. Themethod of claim 17, comprising controlling the swirling of the fluidstream to maintain the radial acceleration at a value at which shearingof the two immiscible phases does not cause an emulsion to form.